This invention relates to a process for manufacturing a copper-clad laminate which comprises a rolled copper foil as a circuit conductive material.
A printed board is a laminate which comprises a substrate made of a porous material, such as paper, glass and woven cloth, impregnated with a thermosetting resin such as phenolic resin, formalin resin and epoxy resin, and a thin copper foil laminated to the substrate. Thus, a printed board is commonly called a copper-clad laminate.
As copper foil for such a copper-clad laminate there has been consistently employed for the past several decades a so-called electrolytic copper foil which is produced by electrolytic deposition from a copper ion-containing solution. One of the reasons for the employment of electrolytic copper foil is that it is extremely difficult to produce a wide and thin copper foil by rolling process. Another is that metallic copper is inherently very poor in adhesion to synthetic resins. Thus, in practice, electrolytic-deposition is conducted such that the adherent surface of the resulting copper foil is roughened to form an electrolytic copper foil. The roughened surface of the copper foil is then electrochemically anodized to form a layer of minute cuprous oxide, or optionally copper oxide, of a few micron in thickness thereon. As a result of roughening followed by anodization, the adhesion of the copper foil is improved for practical use.
FIG. 1 is a cross section of view of an example of oxidized roughened electrolytic copper foil thus obtained. Shown at 1 and 2 are electrolytic copper and copper oxide (hereinafter, among with cuprous oxide, referred to as "copper oxide"), respectively. In general, a copper-clad laminate is produced by a process which comprises applying an adhesive on the roughened surface thus obtained on the copper foil, and then pressing the copper foil with the roughened surface opposed to a prepreg being preimpregnated with thermosetting resine partially curved to B-stage at a proper temperature and pressure, whereby the both are uniformly bonded to each other. In this case, the roughened surface which comprises minute particles of copper oxide serves to hold the adhesive and thus adds to the adhesion or the copper foil to the substrate. The effect of the oxidized roughened surface on the adhesion of the copper foil to the substrate can be clearly recognized in the inventors' study which gave the results that the peel strength of the aforementioned roughened copper foil from a paper-phenol base material to which it is laminated with a butylaric or phenolic adhesive tested in accordance with JISC 6481 reads high as 1.9 to 2.2 kg while that of a plain copper foil having no oxidized roughened surface tested on the same condition reads low as 0.2 to 0.5 kg.
However, a series of studies conducted by the inventors on high frequency electrical transmission characteristics such as a study of printed circuit boards in search for better tone in audio and study or printed circuit boards on resonance characteristic in television picture circuit made it clear that copper oxide which is provided on the surface of the copper foil for better adhesion has an adverse effect on high frequency response and the characteristics of the electrolytic copper foil itself is poorer than that of roller copper foil.
Furthermore, a closer observation of the interface between the copper foil and the substrate in the conventional copper-clad laminate showed that copper oxide particles 2 are deeply embedded in a substrate 3 of synthetic resin as shown in FIG. 2, providing a high adhesion. Accordingly, the conventional copper-clad laminate is disadvantageous in that even after etching following printing of a circuit pattern the copper oxide particles 2 which contain a copper oxide nucleus remain embedded in the substrate 3 as shown in FIG. 3. Such a laminate cannot be used as a circuit unless the particles 2 are removed by a stronger etching which can cause a so-called side etching of the copper foil. Therefore, if the conventional copper-clad laminate were used, conductor width and conductor spacing should be restricted than specified values.
As described above, a copper-clad laminate which employes the conventional oxidized roughened electrolytic copper foil has tow shortcomings associated with high frequency response and etching process for circuit formation.